Many conductive inks include a metal, such as silver, copper or aluminium, in a resin (binder) or resin (binding) medium. While such inks produce upon curing conductors which are substantially conductive and have a comparatively low electrical impedance (or resistance), the resulting conductors are substantially opaque and do not allow the transmission of any appreciable amount of light in the visual spectrum or other important spectra, such as ultraviolet and infrared spectra. However, optically transparent conductors are needed in a wide variety of applications.
Transparent conductors refer to thin conductive films coated on high-transmittance insulating surfaces or substrates. Transparent conductors may be manufactured to have surface conductivity while maintaining reasonable optical transparency. Such surface conducting transparent conductors are widely used as transparent electrodes for example in flat liquid crystal displays, touch panels, electroluminescent devices, and thin film photovoltaic cells, as anti-static layers and as electromagnetic wave shielding layers.
Typical printable transparent conductors, while having reasonable optical transparency, unfortunately often have a comparatively high electrical impedance and low conductivity when cured, with resistances typically in the range of 800-1000 or more ohms per square (e.g., polyethylene-dioxithiophene). Vacuum deposited metal oxides, such as indium tin oxide (ITO), are commonly used industry standard materials to provide optical transparency and electrical conductivity to dielectric surfaces such as glass and polymeric films. However, metal oxide films are fragile and prone to damage during bending or other physical stresses. They also require elevated deposition temperatures and/or high annealing temperatures to achieve high conductivity levels. There also may be issues with the adhesion of metal oxide films to substrates that are prone to absorbing moisture such as plastic and organic substrates, e.g. polycarbonates. Applications of metal oxide films on flexible substrates are therefore severely limited. In addition, vacuum deposition is a costly process and requires specialized equipment. Moreover, the process of vacuum deposition is not conducive to form patterns and circuits, which typically leads in the need for expensive patterning processes such as photolithography.
A transparent conductive coating is needed for touch screens to operate the devices (tablets, smart phones etc.) via finger touch (for example resistive or projective capacitive). For the capacitive touch screens a high resolution pattern of the transparent conductive layer is required. The use of flexible plastic films instead of glass substrates enable a weight reduction and also enable the production of flexible touch screens, what opens a completely new area of applications.
Accordingly, there remains a need in the art to provide transparent conductors having desirable electrical, optical and mechanical properties, in particular, transparent conductors that are adaptable to rigid and flexible substrates and can be processed via simple processes.